There are regulations currently in place in the U.S. that mandate certain retractor locking requirements which specify that acceleration sensors must be operable to lock the retractor at either one of two conditions. The first condition requires that at threshold accelerations of 0.7 g's, the sensor should cause locking of the retractor in any of three mutually-perpendicular planes. In these planes, the retractor must also lock at the specified acceleration when tilted at 45.degree. increments to a range of 180.degree. in the plane.
Automobiles are manufactured with their seat backs at a nominal design and installed position from which they can be shifted and tilted forwardly and rearwardly to respective maximum forward and rearward inclinations of the seat backs. Accordingly, .sctn.209 specifies that if the first condition is not met by the seat back retractor, the other alternative condition that must be satisfied is that the retractor lock when tilted forward or rearward 45.degree. from the design position of the seat back. Where the retractor has the previously-described types of inertia members that may not satisfy the first condition of .sctn.209 in properly detecting vertically aligned accelerations, the second condition where locking must be present at 45.degree. from the design position of the seat back must be established.
One of the difficulties in providing a commercially successful, inclinable retractor being shifted with a changing of the seat inclination is maintaining the sensitivity of the retractor locking mechanism, which typically means keeping the gap between the pawl and ratchet teeth at a constant distance. This is difficult because the inertia weight is often carried in a pivotal support and shifts relative to the pawl, which is mounted on a stationary support. The retractor locking mechanism's sensitivity should be substantially the same when the inertia member is at the nominal seat back position or when swung through almost 45.degree. as when closely adjacent the rearward inclination position.
While there have been a number of prior art acceleration universal sensors designed for use in reclinable seat backs, many of these use inertia members that will not shift properly to sense accelerations in an aligned vertical plane. Further, none of these prior art universal inertia sensors successfully meet the specifications that require locking of the retractor at specified angular range of 45.degree. its position at installation, i.e., its designed position.